This invention relates to Winchester or hard disk digital storage systems, and more particularly to circuitry for powering the head positioner coil. In such storage systems, digital data is stored in the form of magnetic energization on a series of concentric closely spaced tracks on rigid disks having magnetizable surfaces. Data is applied to, and retrieved from, the rotating disks by magnetic heads, which are mounted on a head positioner assembly, and are moved from track to track by the energization of a coil. The coil normally interacts with a permanent magnet structure; and the application of current to the coil in one polarity rotates the head positioner assembly in one direction, to shift the magnetic heads in one direction to different tracks, while current of the opposite polarity shifts the head positioner and heads in the opposite direction.
Now, as the development of Winchester disk drive systems has progressed, more and more data is being stored in relatively small units, with the storage of more than 300 Megabytes of digital information in a standard five-inch drive of the size of a floppy disk drive, now becoming more common. With this higher density of storage, less space is available for cooling fans, heat sinks, or for large inductors or capacitors for filters, for example.
Concerning arrangements for energizing the head positioner coil, most disk drive manufacturers use linear Class A-B power amplifiers in an H-Bridge configuration. Some isolated disk drive manufacturers use linear, Class A head positioner circuitry when the heads are being maintained on a selected track; and when the drive system is switching from track to track, in the "seek" mode, some low frequency switching power amplifier schemes have been employed.
Now, linear Class A amplifiers have good input-output characteristics, having no zero cross-over distortion. However, Class A amplifiers have low power efficiency in terms of power supplied to the head positioner coil, as compared with input power. Class A-B amplifiers are somewhat more efficient, but are subject to zero cross-over distortion as switching from one polarity voltage to another occurs. In each case, the power supply current would be substantially the same as that delivered to the load. Thus, if the voltage supply is at 12 volts, and 1.0 ampere current at 2.0 volts is being supplied to the coil, with the power being 2.0 watts, at least 1.0 amperes current will be drawn from the 12 volt power supply, thus involving 12 watts, and requiring the dissipation of 10 watts in the amplifier. With the substantial power dissipation in the output stages, heat sinking is required.
Low frequency pulse width switching modulation power amplifier circuits are known for the powering of direct current motors, and one prior art patent disclosing such a circuit is M. Gotou, U.S. Pat. No. 4,359,674, granted Nov. 16, 1982. In addition, a switched mode controller circuit for a D.C. Servo Motor intended to be operated in both directions is disclosed in a publication of Unitrode Company, 5 Forbes Road, Lexington, Mass., manufacturer of I.C. chips designated "UC 1637/2837/3637, Switched Mode Controller for D.C. Motor Drive", and an associated application note. However, efficient pulse width modulation switching circuits normally require large size filtering components, such as inductors and capacitors, and such circuits generate switching transients which could interfere with digital data recording and retrieval. In view of the severe space constraints and high frequency noise susceptibility of the low level, high frequency data signals present in Winchester drives to electromagnetic and radio frequency interference, pulse width modulation power amplifiers have generally not been employed in Winchester or hard disk drive digital storage systems.
Accordingly, a principal object of the present invention is to provide a Winchester disk drive head positioner power amplifier circuit which (1) is a linear transconductance power amplifier in its response, (2) has high power efficiency, and (3) does not generate high levels of E.M.I. (electromagnetic interference) or R.F.I. (radio frequency interferences) from switching.